The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell having at least one electrolyte dry-out barrier for restricting transfer of water out of the electrolyte.
Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactants fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. A plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode (xe2x80x9cproduct waterxe2x80x9d) including water resulting from proton drag (xe2x80x9cdrag waterxe2x80x9d) through the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry-out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating and substantial degradation of performance of the fuel cell. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance which could also result in cross-over of the reducing fluid leading to over heating and further degradation of performance of the cell.
Many efforts in fuel cell development have been undertaken to maintain a proper fuel cell water balance and to ensure in particular that a PEM electrolyte does not dry out. For example, it is known to add porous water transport plates adjacent porous anode and/or cathode support layers to facilitate liquid water transport to the anode and/or cathode surfaces of the electrolyte; to form reactant gas distribution channels within the water transport plates in order to facilitate movement of water into the reactant gasses and thereby restrict movement of water out of the electrolyte into the reactant gases; to integrate a humidifying component to add moisture to the gaseous reactant streams entering the cell to limit a possibility of drying out of the electrolyte; to integrate a condensing loop external to the cell to condense moisture within an exiting oxidant stream such as by a heat exchange relationship with ambient air and to then return the condensed moisture to porous support layers adjacent the anode electrode; and, to generate a pressure differential on the anode side of the cell wherein the reactant gases are maintained at a slightly higher pressure than coolant water and anode supply water passing through porous water transport plates and/or porous support layers adjacent the electrolyte, so that the pressure differential assists water transport through the porous support layers toward the electrolyte (as shown in U.S. Pat. No. 5,503,944 to Meyer et al., and assigned to the assignee of the present invention).
These improvements have significantly enhanced fuel cell operating efficiencies. However, PEM fuel cells in particular still suffer operational limits related to dry-out of the electrolyte, especially during long-term operation. Through exhaustive experimentation, it has been determined that usage of zero relative humidity reactant streams entering a PEM fuel cell having a porous water transport plate in fluid communication with the electrolyte eventually causes a drying out of the PEM electrolyte immediately adjacent reducing fluid and process oxidant inlets to the cell. It appears that until the reactant streams are saturated with water, the PEM electrolyte transfers some portion of water into the reactant streams.
Accordingly, there is a need for a fuel cell that can operate with zero relative humidity reducing fluid and process reactant streams passing through the fuel cell without unacceptable drying out of the electrolyte.
The invention is a fuel cell with an electrolyte dry-out barrier, wherein the fuel cell produces electrical energy from reducing fluid and process oxidant reactant streams. The fuel cell includes: an anode catalyst and a cathode catalyst secured to opposed sides of an electrolyte; an anode flow field disposed adjacent the anode catalyst for directing the reducing fluid to pass adjacent the anode catalyst, and a cathode flow field disposed adjacent the cathode catalyst for directing the process oxidant stream to pass adjacent the cathode catalyst; and, an anode electrolyte dry-out barrier secured between the electrolyte and the anode flow field for restricting transfer of water from the electrolyte into the anode flow field. The anode electrolyte dry-out barrier extends from adjacent an entire reducing fluid inlet and along an entire reducing fluid flow path a distance that is adequate for the reducing fluid stream flowing along the reducing fluid flow path to become saturated with water from the anode flow field. The fuel cell may also include a cathode electrolyte dry-out barrier secured between the electrolyte and the cathode flow field for restricting transfer of water from the electrolyte into the cathode flow field. The cathode electrolyte dry-out barrier extends from adjacent an entire oxidant inlet and along an entire process oxidant flow path a distance that is adequate for a process oxidant stream flowing along the process oxidant flow path to become saturated with water.
In a preferred embodiment, the anode and cathode flow fields may be defined by channels in water transport plates and by open pore spaces in porous support and/or gas diffusion layers adjacent the anode and cathode catalysts so that water from the water transport plates moves into the flow fields to saturate the reducing fluid and process oxidant streams. A preferred distance the anode electrolyte dry-out barrier extends along the reducing fluid flow path is at least three per cent of a length of the reducing fluid flow path, wherein the length of the reducing fluid flow path is from a beginning point of the reducing fluid flow path at the reducing fluid inlet through a shortest distance of the reducing fluid flow path through the anode flow field to an end point of the reducing fluid flow path where the reducing fluid leaves the anode flow field. A preferred distance the cathode electrolyte dry-out barrier extends along the process oxidant flow path is at least five per cent of a length of the process oxidant flow path, wherein the length of the process oxidant flow path is from a beginning point of the process oxidant flow path at the oxidant inlet through a shortest distance of the process oxidant flow path through the cathode flow field to a process oxidant flow path end point where the process oxidant leaves the cathode flow field. The anode and cathode electrolyte dry-out barriers may be formed by applying a coating or a film to a porous support, gas diffusion layer or water transport plate between the electrolyte and the respective anode or cathode flow field. The coating or film may consist of dry-out barrier materials compatible with a working environment of a fuel cell, such as a plastic, polymer, elastomer or resin material with low water absorption properties, a ceramic, or a metal. Additionally, the porous support or gas diffusion layer may be impregnated with dry-out barrier materials.
By providing the fuel cell with anode and cathode electrolyte dry-out barriers, the fuel cell may receive very dry reducing fluid and process oxidant streams having a zero per cent relative humidity without fear of drying out the electrolyte adjacent the reducing fluid and/or process oxidant inlets. Where the electrolyte is a proton exchange membrane (xe2x80x9cPEMxe2x80x9d), such drying out may lead to reactant gas cross over and consequent degradation of operation of the fuel cell.
Accordingly, it is a general object of the present invention to provide a fuel cell with an electrolyte dry-out barrier that overcomes deficiencies of prior art fuel cells.
It is a more specific object to provide a fuel cell with an electrolyte dry-out barrier that does not significantly decrease performance of the fuel cell.
It is yet another object to provide a fuel cell with an electrolyte dry-out barrier that can be readily secured to a fuel cell without any significant increase in size of the fuel cell.
It is still a further object to provide a fuel cell with an electrolyte dry-out barrier that may be selectively applied to either an anode or cathode side of the fuel cell.
It is another specific object to provide a fuel cell with an electrolyte dry-out barrier that is inexpensive to manufacture and to secure within a fuel cell.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.